A 510(k) clearance rests on comparison to an existing predicate; a De Novo grant creates a brand-new classification because no predicate exists.
The practical difference shows up downstream: once your device gets a De Novo grant, it becomes a predicate other companies can cite in their own 510(k)s — you've effectively created the classification everyone after you will use. A 510(k) clearance never does that; it just adds one more cleared device under an existing classification. Choosing the wrong one at the start (assuming a predicate exists when it doesn't, or vice versa) means restarting the strategy from Phase 1.
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